On vehicles, especially motor vehicles, disk brakes form probably the most widely used type of brake system. Disk brakes are composed essentially of a brake disk and of a brake caliper which fits around the edge of the brake disk. In this arrangement, the brake disk is connected to the vehicle wheel to be braked by a wheel hub supported rotatably in the axle stub. In contrast, the brake caliper is fixed on the axle stub. The actual deceleration is achieved by means of brake pads, which can be placed against the brake disk and which are arranged on both sides of the brake disk, between the latter and the brake caliper.
Depending on the application, brake disks can be composed either of iron or, alternatively, of carbon-ceramic or aluminum. Although brake disks produced from iron, more specifically from gray cast iron, are very widely used, they have the known problem of surface rust. Moreover, there is sometimes a damaging effect in combination with aluminum rims, into the surface of which the hot iron particles which become detached during the braking process can burn to a significant extent.
Overall, brake disks should have a braking surface which shows as little wear as possible and releases little fine dust. To achieve this, the aim is to make the surface as hard as possible. Thus, in the case of aluminum brake disks, for example, silicon carbide (SiC) may be added, this being deposited as a wear-resistant protective layer on the surface. However, the production of brake disks from non-ferrous materials is in some cases difficult and generally expensive.
Another embodiment of such a protective layer can be achieved using thermal spraying. In this case, the material to be applied to the surface of a main body of the brake disk is softened in advance by the action of heat and is accelerated in the form of individual particles by means of a gas stream. When the particles make contact, a purely mechanical bond (e.g., without fusion of the surface of the main body) is formed. The materials can be metals as well as oxide-ceramic or carbide materials. Apart from high costs, the disadvantage here is especially the durability of such protective layers. Thus, only moderate roughening of the surface by means of sand blasting, especially corundum blasting, is generally possible, but this does not lead to a durable mechanical bond. For example, when using hard gray cast iron for the main body, dovetail roughening, an advantageous roughening process, is not possible.
In a previous application originating from the Applicant, DE 10 2013 221 737.4, a method for producing a brake disk which envisages the arrangement of an oxidic enamel coating on the friction surfaces thereof is disclosed. This coating is baked into the base material of the brake disk in order to achieve a metallurgical bond through phase formation. In its arrangement, the enamel coating serves as an antiwear and anticorrosion layer. However, it should be noted here that the thermal stress on brake disks protected in this way is limited. The reason for this is the glass transition temperature of oxidic enamel coatings of approximately 650° C. If corresponding temperatures are reached by demanding braking operations, said enamel coating can begin to creep and/or to flow. This is due essentially to the high surface pressure of the brake linings coming into contact with the friction surfaces combined with simultaneous softening of the enamel coating.
Thus, it is known that brake disk temperatures caused by braking processes can easily reach a level of 700° C. Current brake disks must generally pass the “Auto-Motor-Sport Test” (AMS). This specifies 10 braking cycles in quick succession, in which a vehicle fitted with the brake disks to be tested is alternately accelerated to 115 km/h and then braked to a halt with a maximum braking force.
DE 10 2005 022 264 A1 and the DE 10 2006 050 985 A1 disclose the possibility of forming a NiCrBSi coating on a steel substrate. For this purpose, mixing the metallic powder with an aqueous enamel slip is proposed, wherein the mixture formed is applied to the surface of the substrate by spraying or immersion, for example. During the subsequent drying process, the water component of the mixture is removed, whereupon the substrate thus prepared is heated to 1040° C. to 1060° C. The sintering of the mixture which is brought about in this way can be accomplished either in a furnace or by open flaming or by means of induction coils. In contrast to other coating methods by simple application, it is essential here that a metallurgical bond should be formed between the coating and the surface of the substrate. This gives a kind of welded joint which, in contrast to otherwise conventional spray coatings, does not allow subsurface corrosion damage.
However, the coating disclosed by the abovementioned publications must be critically considered in connection with brake disks since there are justified reservations relating to health regarding the nickel dust (fine dust) which is released during braking. Thus, such a coating, which would otherwise be advantageous, is unsuitable for brake disks.
Other methods known in the prior art generally envisage the formation of an oxide layer on the surface of the respective component, which may be advantageous. This contributes to an increase in wear resistance and provides cathodic protection from further corrosion. For this purpose, the material of the component itself is subjected to a suitable procedure by means of which the desired oxide layer can be formed on the base material.
Since brake disks are mass-produced wearing parts, they are predominantly manufactured from iron, especially gray cast iron. However, the formation of iron oxide tends to be unwanted here. This is because the formation of an iron-based oxide involves a corrosive process which destroys the brake disk over time. Apart from the deterioration in appearance due to even slight superficial rust, this quite often leads to an acoustic deterioration which manifests itself in unpleasant squealing.
The previously known coatings based on electrolytic or spraying methods do not currently allow the possibility of producing permanently wear- and corrosion-resistant brake disks, especially gray cast iron brake disks. This also applies to the desired optimum friction coefficients of said brake disks for adequate deceleration. Given the prior art cited, there is still plenty of room for improvement in the manufacture of mass-produced brake disks which are more durable overall.
Given this background situation, it is the underlying object of the disclosure to present a method for producing a brake disk and a brake disk for a vehicle which allows low-cost manufacture that is easy to integrate into existing processes, wherein the brake disk itself meets all the requirements for the reliable functioning thereof over a long period of time.